Making pH sensors more durable

01 November 2007

Manufacturers have learned how to make accurate pH sensors, and keep them in calibration. The trouble is, in harsh conditions they don’t last very long—sometimes, only a few hours.

Measuring pH (pondus Hydrogennii, or hydrogen exponent) is important for all industries that use water in manufacturing. pH is the measure of hydrogen ion activity in a solution; the more hydrogen ions there are, the more acidic is the solution.

One of the great difficulties in measuring pH is the extremely wide range in which the sensors must operate:
the pH scale, from 0 to 14, is actually logarithmic, representing 14 orders of magnitude. Thus pH 0 is one mole of H+ ions per litre and at the other extremity of the scale, pH 14 is 10-14 moles of ions per litre. Vinegar and lemon juice have a lot of hydrogen ions and are quite acidic; their pH value is about 2.5. Pure water is pH 7, and bleach is 12.5.

Glass pH sensors were invented 100 years ago by the German scientist Max Cremer. Commercial manufacturing of glass electrodes began in the 1930s, and these were, of course, laboratory instruments. It wasn’t until after the development of the combination electrode (combining the measurement and reference electrodes, 1947) and the preamplifier (1966) that real in situ industrial pH measurements could be made.

The first industrial instruments were analogue. With the progressive use of digital electronics, measuring pH became much easier. But there remained one significant problem: the sensors, however accurate, didn’t last very long. Some industrial processes ate them up.


Endress + Hauser had the right idea when it introduced MemoSens three years ago. It wasn’t so much an innovation in pH sensors as it was the way the sensors are calibrated and connected to the measuring system.

pH measurement is even more difficult in harsh applications, and of course nobody likes to work in hazardous areas. Yet there are known circumstances where the pH sensor has to be calibrated in situ
twice a day, and one calibration can take up to 30 minutes.

E+H addressed the problem in two ways. First, it built a digital sensor that includes memory capability so that it can be calibrated in the laboratory at any time prior to use and kept in inventory. As soon as the electrode is installed, the calibration data are automatically uploaded to the transmitter.

The digital sensor not only reports the pH value, but also diagnostics such as status of the electrode, length of time it has been used in the process, the number of times it has been over 80ºC, and the impedance of the glass electrode, which is an indication of its integrity.

E+H’s second big innovation was the way the sensor connected to the transmitter with an inductive, noncontact plug-in connection and bayonet lock. Not only does it make frequent changes easier, it offers complete galvanic isolation, and goes a long way in solving problems such as moisture ingress, leakages, corrosion, salt bridges, ground loops and handling.


The use of glass pH sensors in some industries is such a problem, the pH measurement is avoided, or supplanted with conductivity readings. Who can blame a soft drink manufacturer for not wanting to introduce the possibility of broken glass in a consumer’s drink?

At Achema last year Bürkert Fluid Control Systems was showing for the first time its new 8201 enamel pH sensor, which is virtually unbreakable and so rugged it doesn’t have to be removed during cleaning-in-place (CIP) or sterilisein- place (SIP) operations as is routinely required for most pH sensors.

Glass electrodes may meet all the hygienic requirements, but they cannot be cleaned in place with the steam or acidic / alkaline cleaning solutions. This means they have to be, removed from the pipelines using a change fixture before they can be cleaned (they also have to be removed while the pipelines themselves are being cleaned). They are then cleaned separately with chemicals in the change fixture and may also have to be calibrated. The risk during this procedure is that opening the process pipelines allows the ingress of germs and impurities into the process.

The 8201 is suitable for measuring absolute pH values in liquids between pH 0 and pH 12 at medium temperatures of up to 140ºC and process pressures up to a maximum of 6 bar.

An enamelled steel pipe is used as the basic carrier. The measuring electrode and reference electrode are combined in one element. The measuring electrode is created by attaching an ion-sensitive enamel layer (yellow in the photo) with metallic voltage conductor (positioned in the non-conductive blue enamel carrier
layer); ion exchange takes place on the surface of this enamel layer. The reference electrode is located in the interior of the enamel pipe filled with electrolyte. Voltage transfer takes place when the electrolyte makes contact with the measuring solution via a special diaphragm in the lower end of the pipe. A Pt1000 sensor for temperature compensation is included.

The smooth enamel surface guarantees sterility for biochemical reactions, and since the sensors don’t have to be removed for CIP and SIP procedures, the risk of contaminating the batch reactor or fermenter is
lessened. Expensive fittings for insertion and removal of the sensor can be dispensed with. Solid particles in fermentation processes are less of a danger for the enamelled electrode.

Another benefit of the sturdy electrode is that it does not start showing age-related signs of wear until after many years of service. This not only lengthens the service life, but also the process reliability, since the recalibrations that have to be performed on a regular basis with conventional pH electrodes are only required at very long intervals.

Bürkert says one or two annual calibrations should be sufficient to maintain the sensor, as well as a yearly
exchange of the electrolyte supply for the reference electrode.


Food processor Archer Daniels Midland (ADM) says it has found Foxboro’s pH sensor can be used for months without replacement in its ethanol production plant in Cedar Rapids, USA.

To accommodate ADM’s application, Foxboro Measurements & Instruments adapted its DolpHin line—introduced five years ago— to develop a fiel replaceable measuring electrode that could withstand severe temperature cycling up to 121ºC.

The measuring electrode features a pH glass formulation that provides better measurement stability, accuracy, and longer service in high-temperature applications. This glass also increases response speed up to five times. The electrodes are available in domed, spherical, or ruggedised flat glass. The domed glass electrode is for the harshest applications: temperatures up to 121ºC and extremes of chemical concentrations.

The spherical glass electrodes are for standard process applications up to 100ºC, and the flat ruggedised glass electrodes are for applications where the process water contains solid materials with pH between 2 and 12 and temperatures up to 85ºC. All electrodes are interchangeable and their plug-in design facilitates quick replacement to address changes in measurement conditions or application.

‘Rebuildable sensors are definitely the most cost effective way to measure pH for our applications,’ says Lloyd Feickert, instrumentation supervisor at the ADM plant. The domed 871PH electrode has increased pH sensor service life from 10 days to four months,’ he says. ‘That’s a 1000 percent increase!

In addition to the product cost savings, ADM has significantly reduced labour and maintenance costs. ‘Every time you send a person out to work on a sensor, it’s at least an hour’s worth of labour. I estimate that we have reduced time spent on changing electrodes over the course of a year from 36 hours per electrode to three hours per electrode,’ he says.


Last year, Emerson Process Management introduced its new generation of high temperature sensors of the PERph-X line, for increased sensor life and greater performance in temperatures up to 145ºC.

John Wright, VP of marketing,
Rosemount Analytical Liquid Division of Emerson Process Management, describes it as ‘an entirely new sensor platform with many specialised design features’ such as a new glass formulation called AccuGlass that resists cracking and maintains near theoretical response even at extreme pH values and after exposure
to very high temperature applications.

They also feature an improved double junction reference that can be refilled for extreme applications that may deplete reference electrolyte. This is a common problem in high temperature applications that can coat, foul or produce large measurement offsets.

Reference flow into the process stream is controlled using a porous Teflon reference junction, which is replaceable for use in dirty or oily applications. The Titanium and Ryton outer body construction is resistant to high temperatures and pressures, and can be used ‘cycle after cycle.’

‘The innovative design behind the new generation high temperature sensors establishes a new benchmark for performance and life in elevated temperature environments. Testing at 145ºC indicated that the new sensors outperformed the competition in response time, sensor lifetime and accuracy,’ said Mr. Wright.

‘While customers will seldom need to use the sensors at these levels, we wanted to demonstrate the significant long life potential of these new sensors in real-world applications, such as pulp and paper, and chemical processing,’ he said.

And so it appears that the industrial pH sensor, one of the most fragile and problematic of industrial instruments, is becoming more adapted to the industrial environment. The leading sensor manufacturers now say their sensors will operate at temperatures well beyond 100ºC, and the calibration frequency has been improved. Most of the recent advances are the results of improved materials: making better glass, for example, or in the case of Bürkert’s enamel sensor, an entirely new concept. For harsh and difficult environments, the pH measuring situation has improved.


Gilead Sciences, an international biopharmaceutical company, launched an effort in 2003 to solve its
greatest instrumentation problem: unreliable pH sensors.

‘Our pH sensors simply could not hold up to aggressive chemicals, such as hydrobromic acid,’ said Rob Pastushak, at Gilead’s facility in Alberta, Canada. The organic solvent constituent caused the probe's O-rings to degrade during the most critical stage of the process. In many cases, three probes, at approximately $600 per probe, would fail while processing just one batch.

Gilead was forced to confirm measurements on a benchtop meter in its lab. ‘When you process 3,000 to 5,000 litres and add 5 to 10 kilos of caustic solution at a time, it might take 20 to 40 lab tests to ensure the pH is right during adjustment,’ says Mr. Pastushak. ‘Going to the lab so often just killed production efficiency.’

Given the competitive nature of the biopharmaceutical manufacturing business, even a small variance in yield can have a huge impact on the bottom line. ‘A 1% or 2% increase in the target commercial yield
translates to 100% profit gain. Likewise, a consistent loss of 1% or 2% of the commercial target yield
translates as lost profit. You don't stay in business long with that type of performance,’ he says.


The company decided to test the Foxboro 871PH Series sensor, from Invensys Process Systems. The 871 is a ‘rebuildable’ pH probe that incorporates patented technology from the Foxboro DolpHin sensor line.

Being rebuildable, the sensor includes a robust and continuously reusable sensor body with a field replaceable measuring electrode, reference junction and electrolyte. The measuring electrode is the
‘business’ area of the sensor and includes glass that comes in contact with the media being measured. The sensor features a patented glass formulation that improves measurement stability, accuracy, and service life in high temperature applications, up to 120°C.

The glass also increases response speed up to five times compared with conventional sensors and allows longer duty cycles. Gilead uses the Foxboro 871PH probe in conjunction with two 7,600-litre reactor vessels stationed sideby- side with a shared condenser.

To ensure that the product comes out of solution with the proper pH, Gilead typically dilutes the organic mixture with water. This mixture must then be measured for pH and adjusted until the right balance is achieved. To adjust the pH, Gilead pump-circulates the solution through the bottom of each vessel to the top where the sensor measures pH in a slurry loop. The probe provides reactive, real-time pH measurements, which are key to reducing cycle time.

‘We can now complete a pH adjustment in three hours rather than the 18 to 24 hours it previously took,’ says Mr. Pastushak. ‘And we no longer have to take 40 samples to the lab to confirm measurement accuracy—we only take one, as a matter of quality assurance protocol. Previously, every time we grabbed a lab sample, we had to put the process on hold until we got the results back.

‘The results have been consistent from batch-to-batch,’ says Mr. Pastushak. ‘As soon as we add a solution to adjust pH, the probe responds immediately and provides the new pH reading. We've found it to be accurate to ±0.03 pH units, which is well within our target limits.’

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